1. Field of the Invention
The present invention relates to devices for the spine and, more particularly, to devices for restricting, limiting and/or stopping motion between adjacent vertebrae.
2. Background Information
As we age various changes can occur in the body especially with respect to the spinal column or spine. For instance, the ligaments of the spine can thicken and calcify (i.e. harden from deposits of calcium), bone and joints may enlarge, bone spurs called osteophytes may form, spinal discs may break down, collapse, bulge and/or herniate. Moreover, one vertebra may slip over another vertebra (spondylolisthesis). Facet joints of the spine can also be affected. The facet joints are found at every spinal level (except at the top level) and provide about 20% of the torsional (twisting) stability in the neck and lower back.
Facet joints are hinge-like and link vertebrae together. They are located at the back or posterior of the spine. Each vertebra has two sets or pairs of facet joint portions that join with two sets or pairs of facet joint portions of adjacent vertebrae. Particularly each vertebra has a pair of upward facing facet joint portions (the superior articular facets) and a pair of downward facing facet joint portions (the inferior articular facets). The pair of superior articular facets of one vertebra is linked to a pair of inferior articular facets of a superiorly adjacent (upper) vertebra, while the pair of inferior articular facets of the one vertebra is linked to a pair of superior articular facets of an inferiorly adjacent (lower) vertebra. Thus, each vertebra is linked to an upper adjacent vertebra by two facet joints and to a lower adjacent vertebra by two facet joints.
The facet joints are synovial joints meaning that each joint is surrounded by a capsule of connective tissue and produces a fluid to nourish and lubricate the facet joint. The facet joints slide or glide smoothly (articulate) on each other and therefore both sliding surfaces are coated by a very low friction, most cartilage. A small sack or capsule surrounds each facet joint and provides a sticky lubricant for the joint. Each sack has a rich supply of tiny nerve fibers that provide a warning when irritated. The facet joints allow flexion (bend forward), extension (bend backward), and twisting motion, while restricting certain types of movement. The spine is made stable due to the interlocking nature of adjacent vertebrae through the facet joints. Each facet joint is positioned at each level to provide the needed limits to motion, especially to rotation and to prevent forward slipping (spondylolisthesis) of that vertebra over the one below.
Facet joints are in almost constant motion with the spine and quite commonly simply wear out or become degenerated in many people. When facet joints become worn or torn the cartilage may become thin or disappear and there may be a reaction of the bone of the joint underneath producing overgrowth of bone spurs and an enlargement of the joints. The joint is then said to have arthritic (literally, joint inflammation-degeneration) changes, or osteoarthritis that can produce considerable back pain on motion. This condition may also be referred to as “facet joint disease” or “facet joint syndrome”.
Moreover, facet joints become especially vulnerable when the intervertebral disc is injured. This is true because as the disc degenerates, the volume of the disc is reduced and the disc space tends to flatten out. The disc therefore loses height and the space between the vertebral bodies becomes narrowed. The flattening disc space provides much less support for the facet joints at that particular level, and the joints become subject to increased stress. Facet joint pain is often felt in the spinal area rather than in an arm or leg.
It is therefore sometimes desirable to restrict, limit or immobilize a spinal facet joint because of disease, injury, trauma or the like.
In view of the above it would thus be desirable to have a spinal device for restricting, limiting and/or immobilizing a spinal facet joint.